Modular multilevel converter implements high-voltage output by using low-voltage devices without the need to directly connect switch devices in series. It avoids the issue of series voltage balancing, and is suitable for high-voltage large-capacity applications, for example, flexible direct-current transmission applications. A schematic diagram of a main circuit topological structure of a three-phase modular multilevel converter (MMC) is Shown in FIG. 1(a), including three phase units, where each phase unit includes an upper bridge arm and a lower bridge arm, and each bridge atm consists of several submodules (SM) and one converter reactor in series. The structure of the submodule is shown in FIG. 1(b), which consists of an IGBT half bridge as a switch element and a direct current energy storage capacitor C in parallel, where the direct current energy storage capacitor C is further in parallel with a discharge resistor R. Each submodule includes three working states, as shown in FIG. 2. In a locking state, the submodule may be charged based on the single-phase conductivity of a diode; in an on state, the submodule may be charged or discharged according; to a direction of a current in the bridge arm; and in an off state, the capacitor of the submodule is bypassed, but the capacitor would discharge slowly because it is in parallel with the discharge resistor. To improve the reliability, each bridge arm may further include a certain number of standby submodules, and the number of working submodules of each bridge arm is less than or equal to the number of submodules in series in the bridge arm.
Before the modular multilevel converter works, the capacitors of the submodules need to be charged to a working voltage, to ensure that a difference between superimposed voltages generated by turned-on submodules in the phase units and a voltage at a direct current side is relatively small when the converter is normally unlocked; otherwise, a relatively high impulse current would be caused, and in a severe case, the switch devices may be damaged. When charged via an alternating current side, the capacitors of the submodules can be charged to the working voltage approximately, and normal unlocking of the converter would not cause an impulse current. However in some special cases, for example, in case of the black-start of a flexible direct current system, the capacitors of the submodules of the modular multilevel converter can only be charged via the direct current side. In this case, before unlocking, the capacitors of the submodules can only be charged to around half of the working voltage, and the converter cannot be normally unlocked. Therefore, it is necessary to employ an appropriate strategy to prevent the impulse current.
“Submodule Capacitance Parameter and Voltage Balancing Scheme of a New Modular Multilevel VSC” (Proceedings of CSEE, 2009, Vol. 29 No. 30, 1˜6) by DING Guanjun et al. mentioned a charging method using an auxiliary power supply (Method 1). In the method, a direct current voltage source close to a rated voltage of the submodules is selected and bridged between the positive and negative electrodes of the direct current side of the converter, and throw-in and throw-off of the submodules are controlled such that the capacitor voltages of the submodules rise to the rated value approximately.
“Method for Starting Flexible Direct-Current Transmission System of Modular Multilevel Converter” (at the patent application stage, Application Publication No.: CN201110100456.1) by TANG Guangfu, Kong Ming, et al. mentioned a starting process of a flexible direct current system, in which an active system at one end charges two converter stations (Method 2). An active end converter (station 1) is first charged uncontrollably to create a direct current voltage, and a station-2 converter is unlocked after voltages of submodules of the two stations become stable. The knife of a bypass resistor of the station 1 is closed after capacitor voltages of the submodules of the station-2 converter become stable, then the station-1 converter is unlocked, and finally, the station-2 converter is connected to the grid in a synchronizing mode.
Disadvantages of Method 1 above lie in that: in a bridge arm/phase charging process of the converter, only one submodule can be charged each time, which requires a certain period of time when there is a current-limiting resistor, and it takes a relatively long time to charge the whole converter. After charging of all the submodules is completed, a submodule charged earlier may have a relatively low voltage due to slow discharge by a parallel discharge resistor. Meanwhile, it is complex to separately charge each submodule, and a complex valve control strategy needs to be set. Disadvantages of Method 2 above lie in that: cooperation of the two stations is required for the starting process, and a severe impulse current may be generated in case of inadequate control. When the station 2 is unlocked, an alternating current limiting resistor is still connected in series in the circuit, which can reduce the over-current of the station 1; however, at the transient of unlocking, the direct current voltage of the station 2 decreases to half of that before the unlocking, and a transient impulse current occurs in the station-2 converter.
The present invention is made in view of the disadvantages of the prior art described above.